Gearless traction machines are generally driving a wide range of electric traction elevators. FIG. 1 and FIG. 2 show a typical gearless traction machine construction existing in the art. A traction sheave 10 is driving a plurality of ropes 12 having one end connected to an elevator car and the other end connected to a counterweight. The sheave 10 is rigidly mounted on a shaft 14 which rotates on bearings 16, 18 mounted in pedestals 20, 22. An electric motor 24 is generally attached to one of the pedestals 22 and drives the shaft 14 and the sheave 10. In order for the machine to be rigid, the pedestals 18, 20 are generally mounted on a massive steel structure 26 called a bedplate. Such prior art construction is displayed in Japanese patent JP2003201082, among others.
The problem inherent to this design is that the ropes 12 often interfere with the bedplate 26 when the diameter of the sheave 10 changes. This interference is shown in FIG. 2 as X when the sheave diameter is increased to the value D. The sheave diameter can vary because ropes can be of different diameters and the sheave diameter is generally a multiple of the rope diameter (approximately 40 times). Therefore, the construction described above is not flexible because certain sheave diameters are prohibited or require a specific steel structure in order to be implemented. The bedplate steel structure 26 is generally a massive welded steel assembly, making this change expensive and undesirable. Another problem is that the dimensioning of the lower steel structure also needs to be changed to accommodate sheaves of various widths. The width of the sheave can vary, depending on the number of ropes 12, which can number between 2 and 10 or more, based upon the total elevator load being moved.
An alternative construction is described in U.S. Pat. No. 4,679,661. This reference discloses a sheave that is “overhung”, meaning that it is not supported at one end. This construction allows any sheave diameter to be used because the ropes do not interfere with any part of the supporting structure. However, this arrangement produces a large bending moment applied on the sheave. Therefore, the main structure needs to be very massive in order to limit deflections and stresses, leading to increased cost.
In other prior art embodiments the motor has a so-called “external rotor” (EP1411620A1, JP2002274770, DE4233759A1) but the pedestals supporting the machine are also mounted on a heavy steel structure that eventually interferes with the ropes. In addition, a major disadvantage of such external rotor construction is that the sheave diameter is dependent of the motor diameter, thus reducing the flexibility of the machine.